Distributive resistive mixer

ABSTRACT

The invention relates to devices comprising field effect transistors to detect the power of an electromagnetic high frequency signal V RF . According to the prior art, the high frequency signal is coupled into the gate G and via a capacitor C GD  into the drain D of the field effect transistor FET, the gate G being biased with a direct voltage V g  which corresponds to the threshold value of the FET transistor. The resulting current at the source S contains a direct current portion I ds  which is proportional to the square of the amplitude of the high frequency signal. The operating frequency of said power detectors is limited to a few gigahertz (GHz) by the discrete arrangement and especially by the predetermined gate length of the field effect transistor. The aim of the invention is to improve a resistive mixer in such a manner that it can be operated at high gigahertz and terahertz frequencies. For this purpose, the resistive mixer comprises a line which has a first and a second electrical conductor having respective connecting contacts so that an electrical high frequency signal can be coupled into the line, the first conductor having a plurality of series-connected voltage-dependent resistor elements (R) and at least one capacitive element (C) being interposed between the first and the second conductor.

BACKGROUND OF THE INVENTION

The present invention concerns a resistive mixer for mixing or detectingan electrical or electromagnetic high frequency signal.

The terahertz frequency range or submillimeter wavelength range which isroughly defined as being between 100 gigahertz (GHz) and 10 terahertz(THz) is one of the last ‘dark’ areas in the electromagnetic spectrum.In that frequency range technically usable, in particular coherentsources and corresponding detectors are hitherto not commerciallyavailable or are commercially available only at low frequencies. Thedevelopments in the last decades have resulted in systems which byvirtue of their complexity however are hitherto used only inexperimentally distinguished areas such as radio astronomy oratmospheric research. Hitherto inexpensive sources and detectors are notavailable for applications in everyday life, that being although the THzfrequency range has intrinsic advantages over other frequency bands inthe electromagnetic spectrum:

-   -   many optically opaque materials are transparent in the THz        frequency range,    -   THz radiation is non-ionising and is therefore deemed to be safe        in the biomedical field,    -   given rotatory, vibronic or libratory molecular excitations have        a resonance frequency in the THz frequency range,    -   THz radiation affords essential items of information about        charge carrier dynamics, in particular in nanostructures, which        play an essential part in future photonic and electronic        components,    -   THz radiation exhibits a lesser degree of scatter compared to        optical frequencies and is therefore suitable in particular for        use in industrial environments in which for example increased        dust formation is involved, and    -   if communication systems are considered higher frequencies        permit greater transmission band widths.

Most purely electronic apparatuses operating in the THz frequency rangeare based on GaAs or InP semiconductor technology. It was shown in theend that SiGe and CMOS semiconductor technologies also result inapparatuses operating at up to 100 GHz. At higher frequencies up to 1THz and above more complex quantum cascade laser systems are usedequally as sources as optoelectronic systems based on femtosecond shortpulse lasers or mixing of two continuous-wave laser sources.

At the present time the THz radiation is detected with heterodynemixers, for example Schottky diode mixers, photoconductive detectors orpower detectors such as for example photovoltaic detectors, bolometersor Golay cells.

All the above-described technologies however involve considerablecomplexity in the source and detector components themselves as well asthe manufacture thereof so that although they are admittedly used in thefield of research and development and in research-related fields ofapplication such as radio astronomy, they are not suitable for massmarkets.

U.S. Pat. No. 4,647,848 discloses a field effect transistor circuitwhich is used as a discrete resistive mixer. The field effect transistoris used to detect the power level of an electromagnetic high frequencysignal V_(RF). A detector circuit from the state of the art is shown inFIG. 1 a) of that application. The high frequency signal is coupled intothe gate G and by way of a capacitor C_(GD) into the drain D of thefield effect transistor FET, wherein the gate G is biased with a dcvoltage V_(G) corresponding to the threshold value of the transistorFET. The resulting current at the source N then contains a dc componentI_(ds) which is proportional to the square of the amplitude of the highfrequency signal. The output signal can be detected as a voltage V_(ds)by way of a corresponding external resistor. In that respect thedescribed receiver circuit is made up of discrete components. The outputsignal is filtered with a low pass filter to suppress the ac components.

In that respect the operating frequency of the power detector describedin U.S. Pat. No. 4,647,848 is limited to a few gigahertz (GHz) by thediscrete arrangement and in particular by the predetermined gate lengthof the field effect transistor.

In comparison the object of the present invention is to provide aresistive mixer which operates at high gigahertz and terahertzfrequencies.

SUMMARY OF THE INVENTION

According to the invention that object is attained by a resistive mixercomprising a line which includes a first and a second electricalconductor having a respective connecting contact so that an electricalhigh frequency signal can be coupled into the line, wherein the firstconductor has a plurality of series-connected voltage-dependent resistorelements (R), and wherein a capacitive element (C) is arranged betweenthe first and second conductors.

The term resistive mixers in accordance with the present invention isused to denote both conventional resistive mixers in which modulation ofthe channel resistance is effected in the quasi-static limit case, andalso plasmonic resistive mixers in which collective charge carrieroscillations (plasmons) are excited in the 2-dimensional electron gas.The plasmons can be weakly damped or overdamped.

The term electrical conductor in accordance with the present inventionis used to denote an electrical connection which conducts the electricalcurrent permanently, temporarily or under given environmentalconditions. That embraces not just conventional conductors such as forexample copper or other metals but in particular also semiconductors. Inaccordance with the invention the conductors can preferably havecombinations of metallically conductive portions and semiconductorportions.

The line formed by the first and second conductors is a high frequencyline, also referred to as a transmission line.

In that way in accordance with the invention there is provided an RCline with distributed voltage-dependent resistors and capacitors. Theresistive mixer according to the present invention comprises asuccession of series-connected elementary voltage-dependent resistorelements and parallel-connected capacitors which by virtue of theirdistributed arrangement respectively locally resistively mix a highfrequency signal coupled on to the line and respectively already locallyproduce a direct current which is dependent on the power of the highfrequency signal. Within the distributed resistive mixer the elementaryvoltage-dependent resistor elements achieve much better high frequencyproperties than for example the individual field effect transistor whichis known from the state of the art and which is used for resistivemixing and whose gate length cannot be reduced as desired by virtue ofthe limitation of the available process technologies.

In an embodiment arranged between a first and a second voltage-dependentresistor element is a respective capacitive element between the firstand second line for providing the complete RC line.

In a preferred embodiment of the invention the voltage-dependentresistor elements and the capacitive element are integrated in a singlesemiconductor element on a single substrate. In that way it is possibleto implement not only reduced dimensions, in particular gate lengths inrespect of the voltage-dependent resistor elements, but it is alsopossible to avoid parasitic capacitances as occur in arrangements withdiscrete components or reduce them to the edge regions of the line.

The term voltage-dependent resistor element in accordance with thepresent invention is used to denote an electrical resistor whose valuedepends on the voltage locally occurring at the resistor. In a preferredembodiment of the invention such a voltage-dependent resistor element isa field effect transistor, wherein for the mode of operation of theelement the semiconductor technology with which the serial arrangementof the field effect transistors is implemented is not an importantconsideration. Examples of such implementations are silicon technology,CMOS technology, etc.

In an embodiment of the invention in which the voltage-dependentresistor elements are field effect transistors each having a source, adrain, a gate and a source-drain channel, the gates of all field effecttransistors are so electrically connected together that the gates formthe second conductor of the line, and wherein the drain of a first fieldeffect transistor is connected to the source of a second field effecttransistor so that the source, the source-drain channel and the drain ofthe first field effect transistor and the source, the source-drainchannel and the drain of the second field effect transistor form thefirst conductor.

In a preferred embodiment of the invention the individual elementaryfield effect transistors which are connected in series asvoltage-dependent resistor elements are embodied by the structuring of acommon gate or a common source-drain channel respectively.

In such an embodiment, as clearly described, a discrete field effecttransistor as is known for example as a JFET from the state of the artand whose effective gate length is determined by the spacing between thesource and the drain is divided by suitable structuring of the metallicstructure or the oxide layer of the gate or semiconductor structuring ofthe source-drain channel into an arrangement of interconnectedelementary field effect transistors. Each individual one of theelementary field effect transistors, within the arrangement, has aneffective gate length which is markedly reduced in comparison with thegate length of the original discrete, undivided field effect transistor.

In the case involving structuring of the gate, the metallisation of thegate is preferably in the form of a periodically repetitive structure.In an embodiment that structure is in the form of a grating.

The term semiconductor structuring in accordance with the presentinvention is used to denote a spatial structuring, in particular aspatially varying doping profile. In an embodiment such structuring ofthe doping profile is in the form of a periodically modulated doping ofthe source-drain channel in the vertical or longitudinal direction.

In a conventional field effect transistor the source and the draincannot be taken just as close to each other as may be desired by virtueof the limitations in the process technology so that the source-drainspacing of a discrete field effect transistor cannot be less than 250nm. In addition that source-drain spacing also limits the possiblereduction in the effective gate length of a conventional field effecttransistor.

In comparison with the arrangement according to the invention it ispossible in particular to structure the gate with a level of resolutionwhich is markedly below 250 nm on that the individual elementary fieldeffect transistors formed by the structuring of the gate have muchshorter effective gate lengths than the overall arrangement. If forexample the gate of a field effect transistor with a gate length of 250nm is structured in such a way that there are ten elementary fieldeffect transistors according to the present invention then each of theelementary field effect transistors has only an effective gate length ofless than 25 nm.

In an embodiment of the invention each of the elementary field effecttransistors has an effective gate length or a length of the source-drainchannel of less than 250 nm, preferably less than 200 nm.

In an embodiment of the arrangement according to the invention in whichthe series connection of the elementary field effect transistors isachieved by division of the source-drain channel of a conventionaldiscrete field effect transistor either by structuring of the gate(structuring of the metallic structure and/or the oxide layer) or thesource-drain channel, the implemented elementary field effecttransistors do not have a source and a drain which are to be clearlyseparated from each other. Rather they are in the form of regions of thesource-drain channel of the overall component which are defined by theirfunction. In particular for example a region of the source-drain channelof the overall component which defines the drain of a first elementaryfield effect transistor at the same time also forms the source of asecond field effect transistor.

It is desirable in that respect if the capacitive element or theplurality of capacitive elements of the RC line is also implemented bysuitable structuring of the gate or the common source-drain channel. Inthat way the structuring not only implements the embodiment of thedistributed field effect transistors in the overall arrangement, butalso the parallel connection of the required capacitors.

In a further embodiment of the invention a series of arrangements whichblend into each other is afforded by a structuring of the gate or thecommon source-drain channel instead of a discrete arrangement ofseries-connected distributed voltage-dependent resistor elements. Thatrepresents the limit case of an arrangement having a very large numberof transistors (N tends towards infinity), wherein the effective gatelength of the individual transistors becomes very short and tendstowards zero. In particular such an arrangement can be achieved by thesource-drain channel of the overall arrangement being provided with aramp-shaped doping profile, wherein the doping of the source-drainchannel continuously decreases or increases between the source and thedrain.

In that respect, in the limit case of a further embodiment of thepresent invention, there are capacitor coatings and voltage-dependentresistor coatings, as are also formed within an electrically long FETchannel. That limit case leads to the generation of plasma oscillationsof a two-dimensional electron gas and thus a magnification of the mixereffect.

In that respect it is desirable if the second line is connected to a dcvoltage source for biasing the gate of the field effect transistors. Inthat way in operation the gates of the distributed field effecttransistors can be biased with a dc voltage in such a way that thethreshold value of the transistors is reached.

As such biasing of the gates however causes leakage currents inparticular in field effect transistors with thin gate oxides, analternative embodiment provides that the field effect transistors arefield effect transistors with a negligibly low threshold voltage, thatis to say so-called zero-V_(T) FETs. In that way leakage currents can bereduced even in the case of very thin gate oxide layers and thesignal-noise ratio of the devices can be improved. The spectralsensitivity and band width of the component is further improved by thereduction in the thickness of the gate oxide layers.

In an embodiment of the invention the first and second conductors areconnected at their ends in opposite relationship to the connectingcontacts to an external matching network to increase the sensitivity ofthe arrangement. The external network permits phase matching between thegate voltage and an external RF drain voltage which is additionallycoupled in.

In a further embodiment of the invention provided on the line, inaddition to the resistor elements and the capacitive elements, are oneor more diodes or other non-linear components in order further toincrease mixer efficiency. In that respect the diode can be providedeither in series with the voltage-dependent resistor elements on thefirst conductor or parallel to the capacitive element between the firstand second conductors.

The resistive mixer element in accordance with the present invention hasa wide area of use. In particular the resistive mixer element can beused as a detector for electromagnetic high frequency radiation in theTHz frequency range. In such an embodiment it is desirable if the lineis connected to an antenna element for high frequency radiation so thatelectromagnetic high frequency radiation received by the antenna inoperation of the detector can be fed into the line.

To detect the power level of the high frequency radiation reaching theantenna element it is advantageous if the output of the line isconnected to a charge coupled device (CCD). That charge coupled device(CCD) integrates the dc output of the mixer and the integrated signal isdetected by way of a suitable electronic read-out arrangement.

In a further embodiment with an arrangement in line or matrix form ofdetector elements (camera, focal plane) the charge coupled device (CCD)is read out sequentially and erased after an integration time.

In an alternative embodiment the resistive mixer according to theinvention is used as a mixer for mixing a high frequency signal in theTHz frequency range with an electrical local oscillator signal. For thatpurpose a desirable embodiment of the detector is one in which thesecond conductor of the mixer in one of the above-described embodimentsis connected to a local oscillator source.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and possible uses of the present inventionwill be apparent from the present description and the related Figures.

FIG. 1 a) shows a resistive mixer according to the state of the art,

FIG. 1 b) shows an equivalent circuit diagram of a resistive mixeraccording to an embodiment of the invention,

FIG. 2 diagrammatically shows the structure of a further embodiment ofthe resistive mixer according to an embodiment of the invention,

FIGS. 3A) and B) show the implementation of a voltage (A) and currentoutput (B) at the drain of a mixer in accordance with an embodiment ofthe invention,

FIG. 4 shows a power detector for electromagnetic high frequencyradiation having a resistive mixer according to an embodiment of theinvention, and

FIG. 5 shows a detector for detecting power and phase of electromagnetichigh frequency radiation having a resistive mixer according to anembodiment of the invention.

In the Figures described hereinafter identical elements or elementswhich perform a similar function are denoted by the same references.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a) shows a resistive mixer as is known from the state of the art.It has already been described in the introductory part of thisdescription.

FIG. 1 b) shows an equivalent circuit diagram which schematicallyillustrates the structure of an embodiment of the resistive mixeraccording to the invention. In the illustrated embodiment of the mixerthe elementary voltage-dependent resistor elements are implemented by aseries of field effect transistors FET1, FET2, . . . FETN, wherein thethree transistors shown by way of example stand for a large number oftransistors, for example 20. Each of the field effect transistors FEThas a source S, a drain D and a gate G. In the illustrated embodimentthe field effect transistors are in the form of junction field effecttransistors (JFETs). The field effect transistors are integrated into ahigh frequency line comprising a first conductor 1 and a secondconductor 2. A high frequency signal V_(RF) can be coupled into the highfrequency line having the conductors 1, 2 by way of two connections 3,4. The first conductor 1 of the high frequency line is formed by thesource S′ of the first field effect transistor FET1, the source-drainchannel of the first field effect transistor FET1, the drain D′ of thefirst field effect transistor FET1, a connection between the drain D′ ofthe first field effect transistor FET1 and the source S′ of the secondfield effect transistor FET2, the source S′ of the second field effecttransistor FET2, the source-drain channel of the second field effecttransistor FET2, the drain D′ of the second field effect transistor FET2and so forth until it ends with the drain D′ of the last field effecttransistor FETN.

The second conductor 2 in contrast connects the gates G′ of theindividual field effect transistors FET1, FET2, . . . FETN or isafforded by a continuous structure forming the gates of the individualtransistors. Arranged between the first conductor 1 and the secondconductor 2 are capacitors C′ arranged in parallel. In the illustratedembodiment the number of capacitors C′ corresponds to the number offield effect transistors N. The first conductor 1 and the secondconductor 2 are connected together by way of an external matchingcircuit 5 at the end of the line, that is to say at the drain of thelast field effect transistor FETN, to adjust an external phasedisplacement and thus to be able to increase the sensitivity of thearrangement.

The mixer DRM (Distributed Resistive Mixer) shown in FIG. 1 b) is anintegrated component which is embodied on a single substrate.

Such an integrated implementation of the distributed resistive mixer DRMis diagrammatically shown in FIG. 2. The structure firstly correspondsto a discrete field effect transistor having a gate G, a source S and adrain D of a gate length of about 250 nm. To be able to implement withthat conventional field effect transistor an RC high frequency linehaving an arrangement of series-connected distributed field effecttransistors, the field effect transistor is ‘broken down’ by thestructuring of the gate G into individual elementary transistors FET1through FET6. For that purpose the thickness of the gate oxide overallis reduced but has thickened portions 7. That periodic structuringprovides that on the one hand there are elementary field effecttransistors FET1 . . . FET6 connected in series, while on the other handthe thickened portions of the oxide layer 6 of the semiconductorcomponent form the capacitors C′. In the context of an equivalentcircuit diagram those capacitors C′ and the voltage-dependent resistorsof the individual field effect transistors FET1 through FET6 areillustrated in the semiconductor structure.

In dependence on the load resistance which is connected to the secondend of the line, voltage and current outputs can be provided for readingout the distributed resistive mixer DRM. If as shown in FIG. 3 theoutput of the line is connected to a high load resistance R_(load) thena voltage output can be embodied at the drain of the last field effecttransistor (FETN in FIG. 1 b)). If in contrast as diagrammatically shownin FIG. 3 b) the end of the line is connected to a low load resistanceR_(load) then a current output is implemented at the drain of the lastfield effect transistor (FETN in FIG. 1 b)).

FIGS. 4 and 5 show the use of a distributed resistive mixer DRMaccording to the invention for detecting the power of an electromagnetichigh frequency signal in the THz frequency range or for mixing the THzsignal with a local oscillator signal LO.

FIG. 4 shows an arrangement with which the power of a high frequencysignal Vrf which is connected to the conductor 2, that is to say thegates G′ of the mixer DRM, is detected. Provided at the drain D′ of thelast field effect transistor of the mixer DRM is a filter network Zhaving a low pass characteristic so that only the dc components areintegrated by a charge coupled device C1 and detected by way of anelectronic read-out arrangement OP. As illustrated hereinbefore that dccomponent is dependent on the power of the high frequency signal Vrfcoupled into the gate G′. The biasing voltage Vbias of the gate can beinterrupted and replaced by an erasure voltage by means of a switch S1so that the detector is reset to zero after exceeding the integrationtime.

FIG. 5 shows a detector arrangement which is expanded in relation to theembodiment of FIG. 4 and which in addition to the high frequency signalRF (V_(rf) in FIG. 4) a local oscillator signal, for example at afrequency which is reduced in relation to the high frequency signal, iscoupled into the gate structure G′ of the distributed resistive mixerDRM. In particular the local oscillator signal can be coupled into thegate structure in the form of an electromagnetic wave, preferably by wayof an antenna element.

Alternatively the mixer can also be used for detecting the phaseposition of an intensity modulation impressed on the high frequencysignal. For that purpose, as the local oscillator signal, the modulationsignal, for example at a frequency of 100 MHz, or a signal coupled tothe modulation signal in fixed-phase relationship, is coupled into thegate structure G′. The amplitude of the alternating current of thedistributed resistive mixer DRM, that is detected with the electronicread-out arrangement OP, is then dependent on the phase position asbetween the local oscillator signal LO and the high frequency signal RF.

For the purposes of the original disclosure it is pointed out that allfeatures as can be seen by a man skilled in the art from the presentdescription, the drawings and the claims, even if they are described inspecific terms only in connection with certain other features, can becombined both individually and also in any combinations with others ofthe features or groups of features disclosed here insofar as that hasnot been expressly excluded or technical aspects make such combinationsimpossible or meaningless. A comprehensive explicit representation ofall conceivable combinations of features is dispensed with here only forthe sake of brevity and readability of the description.

While the invention has been illustrated and described in detail in thedrawings and the preceding description that illustration and descriptionis only by way of example and is not deemed to be a limitation on thescope of protection as defined by the claims. The invention is notlimited to the disclosed embodiments.

Modifications in the disclosed embodiments are apparent to the manskilled in the art from the drawings, the description and theaccompanying claims. In the claims the word ‘have’ does not excludeother elements and the indefinite article ‘a’ does not exclude aplurality. The mere fact that certain features are claimed in differentclaims does not exclude the combination thereof. References in theclaims are not deemed to be a limitation on the scope of protection.

The invention claimed is:
 1. A resistive mixer comprising: a transmission line which includes a first and a second electrical conductor each having a respective connecting contact being connectable to a source of an electrical high-frequency signal so that the electrical high frequency signal can be coupled into the transmission line, wherein the first conductor has a plurality of series-connected voltage-dependent resistor elements, whose resistively depend on the voltage locally occurring at the resistor elements, and wherein a capacitive element is arranged between the first and second conductors, characterised in that the voltaqe-dependent resistor elements are field effect transistors each having a source (S), a drain (D), a qate (G) and a source-drain channel, wherein the gates of all field effect transistors are so electrically connected together that the gates (G) form the second conductor and wherein the drain of a first field effect transistor is connected to the source of a second field effect transistor so that the source, the source-drain channel and the drain of the first field effect transistor and the source, the source-drain channel and the drain of the second field effect transistor form the first conductor.
 2. A resistive mixer as set forth in claim 1 characterised in that the capacitive element is arranged between a first and a second voltage-dependent resistor element.
 3. A resistive mixer as set forth in claim 1 or claim 2 characterised in that the voltage-dependent resistor field effect transistors and the capacitor element are integrated in a single semiconductor element on a single substrate.
 4. A resistive mixer as set forth in claim 1 characterised in that the individual field effect transistors are produced by structuring of the gate and/or the source-drain channel.
 5. A resistive mixer as set forth in claim 1 characterised in that the capacitive element is produced by structuring of the gate and/or the source-drain channel.
 6. A resistive mixer as set forth in any one of claims 1 through 2 characterised in that each of the field effect transistors has an effective gate length of less than 250 nm, preferably less than 200 nm.
 7. A resistive mixer as set forth in any one of claims 1 through 2 characterised in that the second conductor is connected to a dc voltage source for biasing the gates of the field effect transistors.
 8. A resistive mixer as set forth in any one of claims 1 through 2 characterised in that the field effect transistors have negligibly low threshold voltages.
 9. A resistive mixer as set forth in any one of claims 1 through 2 characterised in that the first and second conductors are connected at their ends opposite to the connecting contacts to an external matching network.
 10. A resistive mixer as set forth in any one of claims 1 through 2 characterised in that the transmission line has at least one further non-linear component, being a matching circuit or a filter network having a low pass characteristic or a diode.
 11. A detector for detecting electromagnetic high frequency radiation comprising a resistive mixer as set forth in any one of claims 1 through 2, wherein the transmission line is connected to an antenna element for detecting the high-frequency radiation as a source for an electrical high-frequency signal.
 12. A detector as set forth in claim 11 characterised in that the output of the transmission line is connected to a charge coupled device (CCD).
 13. A detector as set forth in claim 11 characterised in that the transmission line second electrical conductor is connected to a local oscillator signal source. 